Actually, the 907 kg (2000lbs each main tank) low fuel warning is a figure that came about for ETOPS certification for the NG, that's the understanding in my company anyway. I certainly know of 737NGs where the low fuel warning triggers at 1000lbs in the main tank, not sure about the Ryanair ones, so stating that 900kg in each main tank is into the NNC is not strictly true.
Edited for kakatoa, cheers, doh!